1) Field of the Invention
The present invention relates to a technology for uniformly fixing toner on a recording medium.
2) Description of the Related Art
A conventional image forming apparatus including an image transferring and fixing device, which transfers to and fixes on a recording medium an unfixed image on an image carrier, has been disclosed in Japanese Patent Application Laid-open Publication No. H9-114282 (page 5, FIG. 1). This image forming apparatus includes an image carrier, an intermediate transfer belt, a heating roller, and a pressurizing roller. The image carrier is in the form of a belt. The intermediate transfer belt is an endless belt that performs an orbital motion, and is disposed such that it is in contact with the image carrier. The heating roller melts a toner image that is transferred to the intermediate transfer belt, by heating the toner image to a temperature not lower than a melting temperature of a toner. The pressurizing roller comes in contact, through the intermediate transfer belt, with a supporting roller that is disposed on a downstream side of the heating roller and supports the intermediate transfer belt. The toner image on the intermediate transfer belt is transferred to and fixed on a transferring material (recording medium) that is fed in between the pressurizing roller and the intermediate transfer belt.
Another conventional image forming apparatus including an image transferring and fixing device has been disclosed in Japanese Patent Application Laid-open Publication No. H9-230646 (page 7, FIG. 2). This color image forming apparatus includes a rotating developing unit that has four developing units. Each of the four developing units contains toner of yellow, cyan, magenta, and black colors respectively. The rotating developing unit develops an electrostatic latent image on a photosensitive drum. A developed image, i.e. a toner image, is transferred to the intermediate transfer member in the form of a belt. The toner image is transferred to and fixed on a recording medium via the intermediate transfer member between a heating roller and a facing roller.
Still another conventional image forming apparatus including an image transferring and fixing device has been disclosed in Japanese Patent Application Laid-open Application No. 2001-13798 (pages 3 and 4, FIG. 1). According to this image forming apparatus, a toner image on an image carrier is transferred to an intermediate transfer member in the form of a belt. Before the toner image reaches a fixing nip that is formed by a pressed contact of the heating roller and the pressurizing roller with the intermediate transfer member sandwiched between the two, the toner image is heated through the belt by a sheet heater that is provided on an inner surface side of the intermediate transfer member and functions as an image forming substance integrating unit. Thus the toner image is combined by softening or melting, thereby integrating by binding at least a part of it. An integrated toner image is transferred to and fixed on the recording medium at the fixing nip.
However, in the image transferring and fixing device disclosed in Japanese. Patent Application Laid-open Publication No. H9-114282, the whole toner image on the intermediate transfer belt is heated by the heating roller to a temperature not lower than a melting temperature of the toner. Therefore, although at least a part of toner particles are integrated by binding, the toner image tends to get deformed easily. When the toner image is pressurized by the pressurizing roller at the fixing nip, the toner tend to be spread along a surface of the recording medium and be deformed along irregularities (ups and downs) of paper fibers on the surface of the recording medium. As a result, there is a big difference in an area of each toner after pressurizing and fixing, resulting in unevenness of graininess and gloss, thereby deteriorating an image quality.
In the image transferring and fixing device disclosed in Japanese Patent Application Laid-open Publication No. H9-230646, no unit has been provided to integrate the toner on the upstream side of the fixing nip that is formed by the pressed contact between the heating roller and the facing roller with the intermediate transfer member sandwiched between the two. Therefore, an image becomes faded if toner particles that form the toner image fall in recesses in the paper fibers on the surface of the recording medium. Further, since heat cannot be transmitted easily to the toner in the recesses, that portion tends to have defective fixing.
Moreover, in the image transferring and fixing device disclosed in the Japanese Patent Application Laid-open Publication No. H9-230646, all the toner that forms the toner image and the recording medium are heated simultaneously to fix the toner image on the recording medium, which reduces the viscosity of each toner. As a result, at a time of fixing, height of toner is changed by 40% to 80% and the area of each toner does not become even due to irregularities on the surface of the recording medium, leading to the deterioration of the image quality.
In other words, as shown in FIG. 10A, even if each toner particle Tn on an intermediate transfer member 61 is deformed roughly in a similar manner (compression deformation), at the time of transferring to and fixing on a paper (transfer paper) P, the toner particles Tn cannot be pressurized evenly due to irregularities (ups and downs) on a surface of the paper P. As shown in FIG. 10B, in a portion in contact with the recesses, an amount of deformation of each toner particle Tn is small and the spreading of the area is small (width W1) because pressure cannot be transmitted easily. Whereas, in a portion in contact with bulges, an amount of pressure exerted being more, there is a big deformation in a direction of width and the area is spread (width W2).
The difference in the area of each toner due to the irregularities on the surface of the paper, as shown by the widths W1 and W2, leads to a deterioration of the graininess of a half-tone image, particularly in a case of a color image. Moreover, that causes a difference in glossiness of the toner image in an area of the recesses and the bulges, resulting in unevenness in gloss, thereby deteriorating the image quality.
Moreover, since the whole toner image is heated, a portion of the toner in contact with the intermediate transfer member is melted and is adhered firmly to the intermediate transfer member. Therefore, in a portion such as a low density portion where an amount of integrated toner is less, sometimes the toner could not be transferred completely to the recording medium.
In the image transferring and fixing device disclosed in Japanese Patent Application Laid-open Publication No. 2001-13798, since there is a unit for integrating the toner image on the intermediate transfer member, the toner cannot be allowed to fall in the recesses on the surface of the recording medium. However, a toner image on an outer surface of the intermediate transfer member in the form of a belt is heated through the belt (intermediate transfer member) both before reaching the fixing nip as well as at the fixing nip. Therefore, when a surface of the toner image that comes in contact with the recording medium is heated up to a required temperature, a portion of the toner that is in contact with the belt is heated to a temperature higher than the required temperature.
The image transferring and fixing devices disclosed in the Japanese Patent Application Laid-open Publication Nos. 2001-13798 and H9-230646 are similar in heating the whole toner image and reducing the viscosity of all the toner. Therefore, at the time of fixing, the height of the toner is changed by about 40% to 80% and there is a difference in the area of each toner due to the irregularities on the surface of the recording medium. As a result of this, the image quality is deteriorated.